1. Field of the Invention
This invention relates to an optical information recording-reproducing apparatus for recording information on and/or reproducing information from an optical recording medium such as an optical disc.
2. Related Background Art
FIG. 1 of the accompanying drawings schematically shows the construction of an optical disc apparatus according to the prior art. In FIG. 1, reference numeral 21 designates an optical disc which is an information recording medium. The optical disc 21 is placed on a turntable and is rotatively driven by a spindle motor 22. An optical head 23 is provided below the optical disc 21 to be movable in the radial direction of the optical disc and a beam of light is emitted from a semiconductor laser 24 contained in the optical head 23 toward the optical disc 21. The emitted beam of light is collimated by a collimator lens 25, is transmitted through a beam splitter 26 and is imaged as a micro light spot on the optical disc 21 by an objective lens 27. The recording or reproduction of information is effected on the optical disc 21 by the beam of light.
The light reflected by the optical disc 21 passes again through the objective lens 27, and thereafter is separated from the incident beam of light by the beam splitter 26. The reflected light thus separated passes through an anamorphic optical system 29 in which the imaging position differs between two directions orthogonal to each other, and is received by a photoelectric conversion element 30. The photoelectric conversion element 30 comprises a plurality of optical sensors, and the output current of these optical sensors is converted into a voltage by a signal processing circuit 31, and a focusing error signal and a tracking error signal are produced from the converted voltage by an error signal detection circuit 40.
The focusing error signal is for accurately focusing the beam of light applied to the optical disc 21 on the disc. An actuator 28 provided in the optical head 23 is controlled by this focusing error signal, and the objective lens 27 is driven in the direction of the optical axis thereof by this control, whereby focusing control is effected. The tracking error signal is for controlling so that the beam of light may accurately trace information tracks formed concentrically or spirally on the optical disc 21. That is, this tracking error signal is fed back to the actuator 28 and drives the objective lens 27 in a direction across the information tracks, whereby tracking control is effected.
By the way, in an apparatus of the type capable of recording such as the optical disc apparatus as shown in FIG. 1, the light quantity of the beam of light applied to the disc differs between during writing and during reading-out and therefore, the quantity of reflected light from the surface of the disc also differs. Thus, even if the amount of tracking error and the amount of focusing error are the same, the signal gain of a servo system will differ depending on the operation mode of the apparatus, and this has led to the problem of causing a difference in the sensitivity of the servo system. So, generally, it is practised to provide a servo AGC (automatic gain control) circuit for controlling the servo gain constantly and stabilize the sensitivity of the servo system. Also, in an apparatus exclusively for reproduction such as a CD-ROM, the above-mentioned AGC circuit is effective when reflectance is irregular depending on the recording medium.
FIG. 2 of the accompanying drawings shows an example of such servo AGC circuit. In FIG. 2, reference numeral 1 designates a two-division photodetector for detecting a reflected light from the optical disc. Here is shown an example of tracking servo. The two-division photodetector 1 corresponds to the photoelectric conversion element 30 of FIG. 1 and is divided into detection elements 1a and 1b. The detection signals A and B of the detection elements 1a and 1b of the two-division photodetector 1 are outputted to a subtractor 2 and an adder 3, respectively, and the difference (A-B) between the detection signals A and B is outputted from the subtractor 2. On the other hand, the sum (A+B) of the detection signals A and B is outputted from the adder 3. The outputs of the subtractor 2 and adder 3 are divided by a divider 4, which outputs K(A+B)/(A+B), where K is a division coefficient. The difference (A--B) between the detection signals A and B of the detection elements 1a and 1b is normalized by the sum (A+B) of the detection signals A and B which corresponds to the total quantity of received light so that the relative value of the difference may not vary even if the total quantity of received light varies. Accordingly, even if as previously described, the quantity of reflected light from the disc varies during writing and during reading-out, the gain of the servo error signal becomes constant and the sensitivity of the servo system can be stabilized. The output of the adder 3 reflects the quantity of reflected light modulated by an emboss pit preformated on the disc and therefore, by an emboss signal obtained therefrom, the address of an information sector and the starting point thereof are detected.
In the servo AGC circuit shown in FIG. 2, assuming that for example, the number of revolutions of the optical disc is 3600 rpm, to effect tracking servo in which the light spot applied onto the optical disc is made to follow the tracks, a frequency band of about 5 kHz from DC is necessary as the servo error signal. Also, when shift is made from the reproducing operation to the recording operation or from the recording operation to the reproducing operation, the quantity of light applied to the optical disc varies suddenly and therefore, to compensate for it so as not to affect the servo system, the output signal of the adder 3 as the denominator signal of the divider 4 need have a band of several hundreds of kHz.
Further, to prevent the DC-like gain fluctuation of the tracking servo, the DC offset of the output of the adder 3, particularly the drift thereof, need be suppressed to a small value. On the other hand, as the emboss signal, a frequency band of about 100 kHz to about 20 MHz is necessary when for the purpose of sector identification or address identification, an optical disc of 2-7 conversion and pit position recording format is rotated at 3600 rpm as described, for example, in ISO, IEC/JTC1 10090. Further, in the case of the pit edge recording and 1-7 conversion format required in the future to make recording density higher, the frequency band of the emboss signal ranges even from several kHz to several tens of MHz. Thus, heretofore, a wide frequency band from DC to several tens of MHz and a small offset voltage have been required of the adder used in the servo AGC circuit and therefore, manufacturing accuracy has become severe, and this has been a factor which increases the costs of the apparatus.